A thermistor is a thermally sensitive resistor whose primary function is to exhibit a change in electrical resistance with a change in its temperature. These devices typically exhibit a negative (nonlinear) resistance-vs.-temperature characteristic, i.e., resistance decreases with an increase in temperature, and are particularly useful when a large resistance change is needed for a narrow range of temperature. For most applications, the thermistor device is typically driven with a small amount of current. When driven with a larger amount of current, a self-heating effect occurs as an excessive amount of power is being dissipated, which heats up the resistor and makes temperature readings impossible.
Thus, when the thermistor device is driven with a large amount of current, it is called operating in a “self-heated” mode. To sense the presence of a liquid, a self-heated thermistor will be sitting at a certain temperature above the ambient temperature. When it comes into contact with water (or any liquid) the temperature of the thermistor will change due to the change in the thermal dynamics of the system. A parameter of the thermistor, known as the “dissipation constant”, normally expressed in milliwatts per degree C. (mw/° C.), is the ratio of a change in power dissipation in a thermistor to the resultant body temperature change. So, in a particular application where it is desirable to measure presence of a liquid, as in a leak detector, the temperature of the thermistor in self-heated mode, is raised. For example, to raise the temperature of the thermistor by 30 C.°, and given an example dissipation constant of 1.0mw/° C., 30 milliwatts of drive power would have to be added in order to raise the temperature 30 C.° which is easily achievable knowing the voltage and resistance at a given temperature.
As an example of a 5.0 kohm thermistor device operating from a temperature between 4° C. to 40° C., raising its temperature 30 C.° in a self-heating mode would result in the thermistor's temperature ranging from 34° C. to 70° C. This would change the thermistor's resistance from 3404 ohms to 876 ohms. Thus, it is seen that the change in resistance is pretty dramatic, and results in different power levels being dissipated if a constant voltage or current source is applied.
Thus, in a self-heated thermistor measurement circuit, it is advantageous to provide a constant power dissipation at all times. Since the thermistors themselves change greatly in resistance over temperature, there are two disadvantages to not doing so. Firstly, what is a reasonable operating power at one temperature can become potentially damaging to the thermistor at another temperature. Secondly, a changing operating power dissipation may result in a changing sensitivity of the measurement circuit employing the thermistor.
Normally such self-heated thermistor measurement systems employ a thermistor device driven with constant current. To measure a leak, one thermistor may be used to work in almost all applications except for the fact that it is difficult to determine a leak on power up if the ambient temperature is not known. In such a scenario, it is advantageous to utilize two thermistors, one a reference thermistor that is always going to be at the ambient (on air) and another that will potentially be wet. The sensor thermistor is the one sensing the leak and a comparative measurement is taken which is performed typically using an analog circuit according to the prior art. Typically, in the prior art, the two self-heated thermistors are connected in the well-known “Wheatstone” Bridge configuration.
U.S. Pat. No. 4,392,782 is representative of a liquid level control device according to the prior art incorporating two thermistor devices that are operated under constant current conditions.
U.S. Pat. No. 6,543,282 is representative of an air flow apparatus according to the prior art incorporating two thermistor devices with one thermistor device having a temperature maintained above an ambient temperature, and with a sensor thermistor device subject to air flow conditions.
It would be highly desirable to provide a leak detector including self-heated thermistor devices that behave independent of the ambient temperature.
It would further be highly desirable to provide a control system for a leak detection device and a method of operating a leak detection system employing two self-heated thermistor devices.
It would further be highly desirable to provide a digital control system for a leak detection device and a method of operating a digital control system for a leak detection system employing two self-heated thermistors.